1. Field of the Invention
The present invention relates to a synchronous network system and a clock management method wherein one of a plurality of clock signals can be selected to synchronize the communication within a network.
In a synchronous network system for the synchronous digital hierarchy (SDH) network, one of a plurality of clock signals is selected as a master clock signal, and a clock signal is generated in synchronization with the selected clock signal thereby to conduct the communication. In the event that this selected clock signal is deteriorated in quality, normal communication becomes impossible. The communication can be continued, however, by switching to a clock signal having a better quality. It is desired to further improve the reliability of the synchronous network system by switching among a plurality of clock signals in this way.
2. Description of the Related Art
A conventional network equipment in a synchronous network system often is not controlled in accordance with the quality information of the clock signal. In such a case, a clock switching unit selectively outputs a clock signal which has been preset by a maintenance operator. In the case where the clock signal turns off, therefore, a clock signal can continue to be generated in a hold over state, in which the clock signal is the one of the immediately preceding phases, or by an internal oscillator. Nevertheless, a problem arises in that the accuracy of the clock signal is difficult to maintain for the synchronous network because the quality of the clock signal is not judged in the hold over state or in the internal oscillator. To avoid this, another clock signal, other than the clock signal of the hold over state or of the internal oscillator, must be selected. To select the another clock signal, the operator is required to control a clock switching unit. Therefore, the inconvenience cannot be obviated rapidly.
In the event that an off-state of the clock signal is detected, an alarm indication signal (AIS) is applied to a corresponding transmission apparatus. Since the synchronization cannot be guaranteed for the synchronous network when the AIS is generated, however, the corresponding network equipment often cannot accurately receive and detect the AIS. The result is a transmission and receiving operation using a clock signal of low accuracy. This has led to a problem of synchronization failure for the synchronous network as a whole.
An object of the present invention is to provide a clock management method and a transmission apparatus such as a network equipment for a synchronous network system which solves the above-mentioned problems of the. synchronous network.
According to one aspect of the present invention, there is provided a clock management method for a synchronous network system, comprising the steps of selecting a master clock signal from a plurality of clock signals and carrying out the communication between transmission apparatuses using a clock signal generated based on the master clock signal or a clock signal generated in the local apparatus alone, selectively switching and outputting the master clock signal by a clock switching unit based on the quality information transferred with a plurality of the clock signals, carrying out the communication by generating a clock signal from a clock generator based on the master clock signal, setting a variable threshold for judging the deterioration of the quality level based on the quality information transferred with the clock signal, and issuing an alarm in the case where the quality level of the clock signal selectively switched and output by the clock switching unit is deteriorated beyond the threshold.
Also, in the case where the quality level of all of a plurality of the clock signals are deteriorated beyond the threshold, the communication can be continued using the clock signal generated by the clock generator under the hold over state. Alternatively, under this state, the communication can be switched in a manner to use a clock signal generated from an independent clock generator. Further, when the quality of the clock signal output from an external output port is deteriorated, an alarm indication signal (AIS) can be sent out in synchronism with the clock signal of the synchronous network system. Also, a plurality of types of units are mounted, and the unit type is identified so that a clock signal corresponding to a particular unit type can be sent out. For example, it is possible to send out an analog clock signal or a digital clock signal selected as the type of the clock signal in accordance with the unit type.
Also, in the case where all of the quality levels indicated by the quality information transferred with a plurality of clock signals are deteriorated beyond a threshold, a deteriorated clock signal, an alarm indication signal (AIS) or other control signal can be selectively output from the external output port. Also, the alarm indication signal (AIS) from the external output port can be sent out in synchronism with the clock signal on the network. Further, in the case where the transmission apparatus has the function of sending out by changing the quality information of the clock signal, if the quality information of the clock signal received and extracted from the line on one side coincides with the quality information of the clock signal received and extracted from the other side or if the quality information of the clock signal received and extracted from the line on the other side coincides with the quality information of the clock signal sent out to the other line then, a closed loop state is determined and a xe2x80x9cdo not usexe2x80x9d message DUS is sent out with the signal. Also, the valid/invalid state of the switching protection can be set, and in valid state, the process for switching to the original clock signal can be included only when a new alarm is not generated within a switching protection time at the time of restoration after an alarm.
According to a second aspect of the invention, there is provided a transmission apparatus of a synchronous network system, wherein a master clock signal is selected from a plurality of clock signals, and the communication is carried out by a clock signal generated based on the master clock or by a clock signal generated only in the own transmission apparatus, comprising a clock switching unit for switching a plurality of clock signals and applying a master clock signal to a clock generator, a synchronization management table for setting a threshold of the quality level based on the quality information transferred with the clock signal, and a quality determination processing unit for monitoring the quality level of the clock signal selectively switched and output by the clock switching unit and controlling the clock switching unit in such a manner as to switch to another clock signal when the prevailing clock signal is deteriorated beyond the quality level threshold set by the synchronization management table.
The system can also comprise the quality control table for converting the quality information of the clock signal received and extracted into a quality level and comparing it with a quality level threshold in the quality determination processing unit. The synchronization management table can have an area for setting the validity/invalidity of the switching protection at the time of restoration after an alarm is generated for a clock signal being selected. Also, the system has a configuration in which the quality information of a clock signal received and extracted from one line and changed to the quality information of the clock signal sent out to the other line is compared with the quality information of the clock signal received and extracted from the other line. In the case where the two quality information are the same, the loop is determined as closed, and a clock signal with a xe2x80x9cdo not usexe2x80x9d (DUS) message attached thereto is sent out to the other line.